Advanced composites are a part of new generation of engineered materials that can be designed and manufactured with precisely controlled microstructure and chemistry to enhance particular material properties. Some of their unique properties are high strength-to-weight ratios, high stiffness-to-weight ratios, good wear and corrosion resistance, low density, low thermal expansion, high electrical resistivity and good fatigue resistance.
Advanced composites have tremendous potential in many civilian and military applications. Prime among these are advanced aerospace and automotive structures where the need exists for lightweight structural materials.
Fiber reinforced composites are under consideration for use in a variety of automotive and aerospace applications, where they must operate in a vibratory environment. Since these materials may enhance noise and vibration attenuation, it is important that the dynamic mechanical properties (i.e., internal damping and dynamic stiffness) of these materials be characterized. However, a review of composites literature shows that although static properties such as strength and fracture toughness have received considerable attention, the dynamic mechanical properties have not received the same level of attention.
Knowledge of mechanical properties of composites and their constituents is fundamental to analyses and design of fiber composite structures. Though some of these properties are determined by physical experiments, several of them are not readily amenable to direct measurement by testing. In addition, testing is usually time consuming and costly. Composite specimens with specific configurations must have been made prior to testing. Furthermore, parametric studies of the effect of fiber volume ratio on various properties can be made only by conducting an extensive series of tests.
Linear elastic composite micromechanics has been used in the past to derive equations for predicting elastic constants of composites based on the corresponding constituent (fiber and matrix) properties. Linear viscoelastic micromechanics has been used to predict dynamic mechanical properties such as damping and dynamic modulus. Equations derived from such studies provide a quantitative insight into the behavior of composites. The various equations can be used to conduct parametric studies as well as sensitivity analyses to assess the effects of using various constituent materials.
Due to the difficulty in measuring fiber properties, such properties have been inferred by substituting measured composite and matrix properties into theoretical micromechanics models. It is evident that accurate properties of the constituents are required to predict composite properties using micromechanical equations. However, very little information is available regarding fiber properties and experimental methods used to measure these properties as a function of temperature. This is particularly true for dynamic mechanical properties.
Characterization of the vibration damping properties of fiber reinforced composites is important for several reasons. In order to design materials with predetermined damping, strength and stiffness properties, it is necessary to measure the dynamic mechanical properties of the composite constituents. Damping in composites involves a variety of energy dissipation mechanisms which depend on vibrational parameters such as frequency and amplitude and environmental conditions such as temperature and moisture. Generally speaking, fibers used for reinforcement of structural composite materials are the primary source of composite stiffness. Thus, data for such fiber properties as dynamic modulus and damping are required by design engineers to understand and predict the dynamic response of composite materials that are subject to impact and vibratory loading. In addition, damping measurement can be a very sensitive non-destructive evaluation tool for understanding and monitoring damage, defects, degradation and time-dependent deformation mechanisms within a material's microstructure.
Test methods for the measurement of mechanical properties of high modulus reinforcing fibers are not as well developed as the corresponding test methods for high modulus composites. For example, the only published standard appears to be ASTM D3379, which describes the static test method for determining longitudinal tensile strength and Young's modulus of single-filament materials.
Single filament experimental techniques to determine interfacial shear strength and failure mode in composite materials have been developed. Kawabata has developed new equipment for the static measurement of longitudinal, transverse and shear moduli of single fibers up to about 400.degree. C. Kawabata, S., "Measurements Of Anisotropic Mechanical Property And Thermal Conductivity Of Single Fiber For Several High Performance Fibers", 253-262, Pro. 4TH Japan--U.S. Conference on Composite Materials, Technomic Pub. Co., Lancaster, Pa. (1989).
Information on dynamic mechanical testing of fibers at elevated temperatures appears to be very scarce. The dynamic extensional modulus of fibers has been measured using forced vibration, non-resonance methods. The principle of these techniques is to apply a sinusoidal extensional strain to the specimens, and simultaneously measure the stress. The viscoelastic behavior is then specified from the relative amplitudes of the stress and the strain, and from the phase shift between them.
DiCarlo, J. A. and Williams, W., "Dynamic Modulus and Damping of Boron, Silicon Carbide and Alumina Fibers," NASA TM 81422:E345 (1980) have developed a cantilever fiber test to measure the dynamic flexural modulus and damping of boron and silicon fibers in flexural vibration up to 800.degree. C. The basic test technique consists of the forced flexural vibration of cantilevered fibers in a high-vacuum cryostate furnace. The tests were conducted in vacuum to eliminate air damping due to large amplitude of fiber specimens when excited. The test specimens were clamped between two stainless steel plates that contained indentation grooves to clamp fibers of various diameters. Dynamic modulus was calculated from the resonant frequencies at which maximum specimen amplitude was observed. In this study, the flexural damping capacity, which is the percentage of stored mechanical energy lost to heat per cycle of specimen vibration, was determined by disconnecting vibrations to freely decay.
The need to experimentally determine the dynamic mechanical properties of constituent materials of a composite in order to build and validate micromechanical models cannot be over-emphasized. From the literature survey, one can conclude that the test methods to measure dynamic properties of composite and matrix materials are well developed, while the test methods to determine dynamic properties of fibers need more attention. This is particularly true for elevated temperature testing.
In a composite, the primary function of the fibers is to support longitudinal loading, and the importance of fiber flexural properties is not clear. In the composite, the load transfer from matrix to fibers occurs by interfacial shear, and the resulting fiber stress is a longitudinal normal stress. Although the flexural vibration of fibers is an accurate test method for determining dynamic properties, one cannot assume that the results are identical to those obtained by subjecting the fibers to longitudinal vibrations along their axes. It is not possible to convert flexural into axial data mathematically without analyzing the distribution of phases within the fiber and understanding their properties.
It is clearly evident that the accurate measurement of constituent material properties as a function of temperature would result in very useful data and composite micromechanics can then be used to predict composite properties and conduct parametric studies easily. This would also mean that by using measured properties of fiber, matrix and composite in micromechanics models, the interphase properties can be deduced. Up to now, the back-calculated fiber properties have probably included interphase effects as well.